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SubjectRe: [PATCH] BUG: nr_phys_segments cannot be less than nr_hw_segments
On Thu, Oct 02 2008, James Bottomley wrote:
> On Thu, 2008-10-02 at 18:58 +0200, Jens Axboe wrote:
> > On Thu, Oct 02 2008, James Bottomley wrote:
> > > The bug would appear to be that we sometimes only look at q->max_sectors
> > > when deciding on mergability. Either we have to insist on max_sectors
> > > <= hw_max_sectors, or we have to start using min(q->max_sectors,
> > > q->max_hw_sectors) for this.
> >
> > q->max_sectors MUST always be <= q->max_hw_sectors, otherwise we could
> > be sending down requests that are too large for the device to handle. So
> > that condition would be a big bug. The sysfs interface checks for this,
> > and blk_queue_max_sectors() makes sure that is true as well.
>
> Yes, that seems always to be enforced. Perhaps there are other ways of
> tripping this problem ... I'm still sure if it occurs it's because we do
> a physical merge where a virtual merge is forbidden.
>
> > The fixes proposed still look weird. There is no phys vs hw segment
> > constraints, the request must adhere to the limits set by both. It's
> > mostly a moot point anyway, as 2.6.28 will get rid of the hw accounting
> > anyway.
>
> Agree all three proposed fixes look wrong. However, if it's what I
> think, just getting rid of hw accounting won't fix the problem because
> we'll still have the case where a physical merge is forbidden by iommu
> constraints ... this still needs to be accounted for.
>
> What we really need is a demonstration of what actually is going
> wrong ...

Yep, IIRC we both asked for that the last time as well... Nikanth?

--
Jens Axboe



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